$node   = [ 'OP', '+' ];
  $stream = [               # AoA
    [ 'VAR', 'x' ],
    [ 'VAL',  3  ]
  ];

  $node   = [ 'VAR', 'x' ]; 
  $stream = [ 'VAL',  3  ]; # not AoA

  $node   = 'VAL';
  $stream = 3;              # scalar